1. Field of the Invention
The present invention relates to an objective lens which is to be disposed in a tip end portion of an endoscope, more particularly to an objective lens for endoscope (hereinafter, often referred to as endoscope objective lens) in which an optical-path deflecting prism is placed on the side of the image-side surface, and also to an imaging apparatus for endoscope using it.
2. Description of the Related Art
In the related art, as a direct-view type endoscope using a solid-state image pickup device such as a CCD (Charge Coupled Device), an endoscope in which a solid-state image pickup device is placed in a tip end portion parallel to the axial direction is known. In such an endoscope, usually, a prism for deflecting the optical path is inserted and placed via a cover glass and the like between the solid-state image pickup device and an objective lens. The size of the optical-path deflecting prism depends on the image size. In order to insert the optical-path deflecting prism, therefore, the distance between the final surface of the objective lens and the imaging position, i.e., the back focus must be sufficiently ensured.
By contrast, in accordance with miniaturization of a solid-state image pickup device, also the image size is reducing. When a sufficient margin is not provided to the distance between a wall surface of the prism and the effective light flux, however, flare or ghost is caused. When the processing accuracy of components and the assembly accuracy are considered, the distance cannot be extremely shortened, and the size of the prism is hardly reduced in proportion to the image size. Therefore, it is requested to develop an objective lens having a back focus which is sufficiently long with respect to the focal length of the objective lens.
In an endoscope objective lens, in order to cope with the request for a wider angle, however, the focal length tends to be shorter even when the image size is identical, and therefore it is difficult to obtain a sufficient back focus. The assignee of the present invention has proposed an endoscope objective lens in which, in a tip end portion of an endoscope, an optical-path deflecting prism can be inserted and placed between a solid-state image pickup device placed parallel to the axial direction and the objective lens, and which has a back focus that is sufficiently long with respect to the focal length (JP-A-10-260348). JP-A-10-260348 discloses a configuration in which a front-group divergent lens system and a rear-group convergent lens system are placed on both sides of an aperture diaphragm, respectively. In the configuration, particularly, optical elements of the front-group divergent lens system are set so as to satisfy predetermined conditional expressions, so that, while aberrations are corrected, a back focus which is three or more times the focal length of the objective lens can be ensured.
In the configuration of JP-A-10-260348, however, correction of lateral chromatic aberration is insufficient as compared with that of axial chromatic aberration, and therefore the correction balance between axial chromatic aberration and lateral chromatic aberration is requested to be improved. Recently, as the high resolution of a solid-state image pickup device is further advancing and the number of pixels is more increasing, it is further required to sufficiently correct lateral chromatic aberration. Therefore, it is requested to realize an endoscope objective lens in which both of axial chromatic aberration and lateral chromatic aberration are satisfactorily corrected while ensuring a back focus having a sufficient length that allows insertion placement of an optical-path deflecting prism.